The present invention relates to man-made implants which replace bones in mammals and more particularly to implants which replace major load bearing sections of bones.
Towards the end of the nineteen-sixties it had been realized that the survival rates of implants anchored via a layer of polymethylmethacrylate bone cement will be limited by the inherent properties of this so called PMMA. During the nineteen-seventies, the search for other modes of implant fixation led to new implant systems. In the United States, several types of porous structures found widespread applications.
The much improved long term success rates of these and other implant systems, in particular of bone and joint replacements, have lead to a reevaluation of the modes of failure:
1. All metal implants are much stiffer than the tissues they replace. This mechanical mismatch has long been considered to contribute significantly to incomplete integration of implants with the surrounding bone tissue. The disuse atrophy observed around many implant systems is regarded as one of the major results of these discontinuities. PA1 2. The necessity for the consideration of systemic effects of metals which previously had been regarded as sufficiently biocompatible has been realized recently from statistical evaluations based on nationwide cancer record systems in New Zealand and Finland. Studies of tissue samples from the vicinity of retrieved titanium alloy implants presented at the Orthopaedic Research Society and the American Academy of Orthopaedic Surgeons Conferences in New Orleans, La. in February 1991 showed that even this titanium alloy does stimulate reactions not much more favorable than those observed around implants made of the cobalt-based alloys.
To overcome the first of these difficulties, the use of isoelastic implants had already been proposed more than ten years ago. As polymers only can match the elastic properties of bone and other kinds of tissue, but do not provide the mechanical strength necessary for load bearing implants, different kinds of fiber reinforcements have been studied in much detail during the last decade in the United States and other countries. Now, with the realization of the systemic effects of long term metallic implants, these polymer implants gain a much higher importance than intended originally. Some may regard them as the last hope for a reliable further extension of implant survival rates. But the success rates of the previous isoelastic implants have remained unconvincingly low.
Most permanent, load bearing implants have a much higher stiffness than the tissue they replace or to which they are intended to transmit loads. This is particularly true for the anchorage portions of joint replacements in the lower extremities. The necessity to reliably stabilize such implants in the bony structures adjacent to the joints which actually need replacement, demands relatively linearly extended anchorage portions. In many cases, the load transfer from the bony tissue to the implant is confined to only small portions of the interface between the bone and the surface of the implant. Because of the large stiffness difference, shear movements result along the interface between the surface of the implant and the adjacent surrounding bone tissue. Such shear movements cause the adjacent bony tissue to transform into soft tissue in a manner similar to the formation of a pseudarthrosis seam as often seen in insufficiently stabilized fracture sites. As such seams are known to increase the probability of a progressive loosening of the implant leading to eventual removal of the implant, the avoidance of the formation of such soft tissue interlayers has become the generally adopted aim. Instead, one tries to achieve and maintain a close bone contact along all interfaces available for load transmission.
Since the differences of the stiffnesses of bone and implant had been regarded as one of the main causes for implant failures, it had been suggested to adjust the stiffness of such implants to that of the surrounding tissue. However, the clinical applications of such "isoelastic implants" have not resulted in improved success rates.
These known isoelastic implants carried means for load transmission to and from the surrounding tissue along all the surfaces of their anchoring portions. Immediately after insertion, however, different portions of the implant are in contact with differently structured bony tissue with different interfacial conditions of load transfer. Thus, the remodelling of the bony tissue will progress differently at different locations of the interface. If for example, because of some locally particularly favorable conditions, the interfacial remodelling leads to the formation of a well load transmitting bond along the middle of the anchoring portion of the implant, one of the remaining parts (the "downstream" or distal one if looked at from the direction of the applied load) will remain unloaded and, thus, not deform with the tissue adjacent to it. This, in turn, would allow for interfacial motion with all the detrimental effects discussed above. It would defeat the intended function of the isoelastic implant.
If isoelastic implants are to perform their intended function, a way must be found to provide for reliable bone contact with the isoelastic implant along all of the interfaces essential for load transmission.